I'm not talking longest actor tenure. I'm saying that taking into account all of the televised minutia concerning age and all of that with The Doctor, do we know which incarnation has lasted him the longest length of time? Strictly from his linear, in-universe perspective, which body has he had the longest?
Either 8 or 1.
All of these dates figure forward from the Doctor's birth.
Assuming no pre-Hartnell incarnations, he lived to be roughly 450 years old. (The second Doctor says that he's 450 in "Tomb of the Cybermen," and that's a year or two after the regeneration.)
The fourth Doctor is about 750, so there's 300 years between "Tomb of the Cybermen" and "Pyramids of Mars."
Another 150 years pass between "Pyramids of Mars" (fourth Doctor) and "Revelation of the Daleks" (sixth Doctor). "Trial of a Time Lord" reveals that the sixth Doctor lives for another fifty years.
The seventh Doctor is 953 in "Time and the Rani."
The eighth Doctor is 1012 in
Vampire Science, and at that point he's already been three years into that incarnation. He's 1200 in
The Dying Days (though everyone believes that book takes place
during Vampire Science). The eighth Doctor spends 120 years living alone on Earth when he's about 1020. He then sleeps another 100 years in
The Sleep of Reason, and that takes us near to the end of the EDAs.
Now, here's where things get tricky.
If The Infinity Doctors stars the eighth Doctor on a reborn Gallifrey, that's several hundred years
after The Gallifrey Chronicles; the Time Lords are well aware that the Doctor traveled the cosmos and then returned home. In that case, the eighth Doctor lives to be at least 2000 years old, giving the eighth Doctor a lifespan of at least a millennium. (Which is neat; the Doctor with the least amount of screentime has the longest life.)
The ninth Doctor says he's 900, which is clearly incorrect. I actually use this to support the
Infinity Doctors theory; the Doctor started counting his life forward from Gallifrey's rebirth. The reason is simple; it doesn't make him look "special." As far as the new Time Lords are concerned, he's the same age as they are.
We have no idea how long the ninth Doctor existed, but he's been around a while; he's the one that fought the Time War.
The tenth Doctor? His incarnation is between 12 and 15 years old by the time of "Journey's End." (
The Infinite Quest itself eats up eight years of solo adventuring.)
The eighth Doctor probably has the longest lifespan, followed by the first (again, assuming no pre-Hartnell incarnations), then the fourth, second, sixth, seventh, fifth.
The one thing we can be sure of? The third, in all likelihood, had the shortest lifespan.